The answer is "it depends"
What kind of fish do you have/plan to have? If stocking cool water fish, e.g. trout you will have problems in Texas regardless of depth.
More important than temp, at least for the commonly stocked warm water species, is the consideration of DO, dissolved oxygen content, of the pond. The higher the temps, in general, the more concerned you need to be about DO.
This is a somewhat complex topic that can take us into a discussion of aeration, thermoclines, nutrient content, stocking rates, fertilizers, etc. There are several folks on here that can really drill down into those topics should you desire to do so.
For the "tree top" view, 15 feet is entirely adequate, so is 12 feet and 8 feet. Just remember to watch your water carefully in summer, especially on cloudy days that have followed long spells of hot weather. DO levels can plummet in a hurry leaving you with problems which can include total collapse and the death of your fish.
Search the forum for those topics, especially fertilizer as it is one of the highest contributors to this catastrophe from the standpoint of what we pond owners can inadvertantly do to ourselves and our ponds.
